1. Field of the Invention
This invention relates to a semiconductor light-emitting device and a manufacturing method for the same. More particularly, this invention relates to a semiconductor light-emitting device of a ridge type having a current blocking layer at a specific portion and a manufacturing method for the same. The invented semiconductor light-emitting device is suitable for a semiconductor laser. In use of a structure of the semiconductor light-emitting device according to the invention, currents can be adequately squeezed, and production yield can be improved during cleavage and assembly. Besides a semiconductor light-emitting device having a high output and a longer life span can be provided when the device with this structure is assembled in a junction-down type.
2. Description of Related Art
FIG. 4 shows a structure of and a manufacturing method for a conventional ridge waveguide type stripe structure semiconductor laser device.
Here, and in the following claims, xe2x80x9cconductive typexe2x80x9d means the same as xe2x80x9cconductivity type.xe2x80x9d As shown in FIG. 4(a), first, a first conductive type cladding layer 11, an active layer 12, a second conductive type cladding layer 13, and a second conductive type contact layer 14 are grown on a substrate 21. Secondly, the second conductive type contact layer 14 and the second conductive type cladding layer 13 are etched as shown in FIG. 4(b) to form a ridge portion. During this step, portions other than the ridge portion are etched to remain a prescribed thickness of the second conductive type cladding layer 13 located on the active layer 12. Protective film 31 made of an insulator is formed on the ridge portion side surfaces and surfaces other than the ridge portion to stop the current flowing. Lastly electrode 32 is formed on an epitaxial side including an area above the ridge portion and electrode 33 is formed on a substrate to produce the device (FIG. 4(c)).
With such a structure, the currents are injected into the active layer 12 via the ridge portion, and the generated light corresponding to the composition of the active layer 12 is generated at the active layer 12 below the ridge portion. Since the protective film 31 having a smaller refractive index than semiconductor portions is formed, the/effective refractive index of an active layer located below areas other than the ridge portion is smaller than the effective refractive index of the active layer below the ridge portion. Consequently, the generated light is confined in a light waveguide below the ridge portion.
Because in this ridge waveguide type stripe structure semiconductor light-emitting device, the ridge portion is formed by etching, it is difficult to control the thickness of the cladding layer portion of a non-ridge portion. Consequently, slight differences of the thickness of the cladding layer of the portions other than the ridge portion cause large fluctuations of the effective refractive index of the active layer of those portions. The width of the ridge bottom, which decides the width of current injection, is not controlled, and therefore, it is difficult to produce lasers having a low threshold and a constant off-angle with good reproducibility.
To solve such problems, a method has been proposed in which the thickness of the cladding layer over the active layer is decided by the crystal growth rate during a crystal growth, in which an isolation layer is formed on areas other than the ridge portion, and in which the ridge portion is re-grown at the ridge portion (e.g., Japanese Unexamined Patent Publication No. 5-121,822, Japanese Unexamined Patent Publication No. 9-199,791). FIG. 5 shows a structure and a manufacturing method for such a laser device. First, after a first conductive type cladding layer 11, an active layer 12, and a second conductive type first cladding layer 13 are grown on the substrate 21 (FIG. 5(a)). Secondly, the surface of the second conductive type first cladding layer 13 is covered with a protective film 31 such as SiO2 or the like, and a second conductive type second cladding layer 13a and a second conductive type contact layer 14a are selectively grown only at a stripe region after a stripe shape window is opened by a photolithography method (FIG. 5 (b)). Lastly, a protective film 31a such as SiNx is formed on side surfaces of the second conductive type second cladding layer 13a, the entire surface of the second conductive type contact layer 14a, and a protective film 31 covering areas other than the ridge portion. The SiNx protective film located on the top of the ridge portion is removed by a photolithography method. And electrode 32 is formed on the whole epitaxial surface and electrode 33 is formed on the substrate (FIG. 5(c)).
Where such a protective film or the like makes the current squeezing, there raise problems that cleavage cannot be made easily due to the surface covered by the protective film as well as peeling off of the electrodes may occur. When the protective film has pinholes or the like, currents may flow through portions other than the ridge portion and there raises a problem that the current cannot be injected at the ridge portion. When the device is assembled in a junction-down type, in which the substrate side is upper while the epitaxial layer side is lower, a soldering material may reach the compound semiconductor layers over the electrodes and the protective film, and there rises a problem that the current leakage may occur easily. Because the ridge portion is projecting from other regions, the device may be easily stressed and degraded. These are not in a favorable situation. The specification of U.S. Pat. No. 5,399,855 discloses a semiconductor light-emitting device having a dummy ridge structure higher than the ridge structure on both sides of the ridge structure. But since the dummy ridge structure is covered by an insulation layer, the above problems regarding cleavage or peeling of electrodes remain unsolved.
With the conventional ridge type waveguide structure semiconductor light-emitting device, even where produced by re-growth, the LD (laser diode) in which currents are squeezed by means of the protective film or the like, cleavage and assembly are not easy, and the currents may not be squeezed adequately by the protective film or the like, so that the production yield may be lowered. When the device is assembled in the junction-down type, the device likely invite the current leakage to areas other than the ridge portion or degradation due to stresses, so that the device hardly obtains adequate LD characteristics.
It is an object of the invention to provide a semiconductor light-emitting device with a high production yield since readily cleaved and assembled, with adequately squeezed currents, and with, when assembled in the junction-down type, a high output and a longer life.
This the inventors have found that, as a result of diligent researches for accomplishing the above object, the above discussed problems are solved by a semiconductor light-emitting device having a structure formed with a semiconductor layer as a current blocking layer outside a protective film formed on both sides of a ridge portion, and completed this invention.
That is, first, this invention is to provide a semiconductor light-emitting device, where including a substrate, a first compound semiconductor layer including an active layer formed on the substrate, a second compound semiconductor layer of a ridge type formed on the first compound semiconductor layer, and a protective film formed above the first compound semiconductor layer on both sides of the second compound semiconductor layer, has a current blocking layer formed above the first compound semiconductor layer outside the protective film.
Secondly, this invention is to provide a method for manufacturing semiconductor light-emitting device comprising the steps of: forming a first compound semiconductor layer including an active layer on a substrate, and a protective film on a center portion of the first compound semiconductor layer; forming a current blocking layer on the first compound semiconductor layer except the center portion; forming an opening in the protective film; and selectively growing a ridge type second compound semiconductor layer at the opening.
Thirdly, this invention is to provide a method for manufacturing semiconductor light-emitting device comprising the steps of: forming a first compound semiconductor layer including an active layer on a substrate and a current blocking layer in this order; removing a portion of the current blocking layer; forming a protective film having an opening at the removed portion; and selectively growing a ridge type second compound semiconductor layer at the opening.
According to this invention, with respect to the semiconductor light-emitting device having this ridge type waveguide structure, production yield during cleavage and assembly processes is improved because of no isolation layer for the purpose of squeezing the current, and the current can be adequately squeezed into the ridge portion since the current blocking layer exists outside the ridge opening. When the device is assembled in a junction-down type (see, FIG. 11(b)), the device can reduce the stresses to the ridge portion and achieve a high output and a longer lifetime, because the outside of the ridge portion is thicker than the ridge portion.
The semiconductor light-emitting device according to the invention can be used as a high output semiconductor laser by widening the width of the ridge bottom, and alternatively, can be used as a self-excited oscillation type semiconductor layer by narrowing the width of the ridge bottom, which can be used as one of preferable examples of the invention.